1. Field of the Invention
This invention relates to a process for purifying a crude aqueous phosphoric acid solution containing a pentavalent vanadium compound by an organic solvent extraction process. More particuarly, this invention relates to a process for preparing pure phosphoric acid, by an aliphatic alcohol solvent extraction, from crude aqueous phosphoric acid containing a pentavalent vanadium compound resulting from acid decomposition of phosphate rock, or from oxidation of crude phosphoric acid prepared by the wet process.
2. Description of the Prior Art
Phosphoric acid is manufactured by two methods: the wet process, in which phosphate rock is decomposed with a strong acid e.g. sulfuric acid, and the dry process, in which phosphate rock is calcined with coke and silica in an electric furnace. In general, the phosphoric acid prepared by the wet process contains various impurities which were present in the phosphate rock and said process has been primarily used for the preparation of fertilizers. The phosporic acid prepared by the dry process is of high purity and has been used in foods, medicines, pure chemicals and the like. Recently, various purification processes, such as organic solvent extraction or the like have been proposed for purifying crude phosphoric acid, prepared by the wet process, for ultimate use in foods, medicines, pure chemicals and the like.
In general, phosphate rock contains impurities such as Ca, Fe, Al, F, Si, etc. Considerable effort has been directed to removing impurities by many different methods. However, the separation of vanadium components has not been substantially investigated. The amount of vanadium components in phosphate rock differs depending upon the type of phosphate rock, and usually is about 150 - 250 ppm. Most vanadium components in phosphate rock are transferred to the phosphoric acid in the wet process. The vanadium components cause contamination by imparting to the pure phosphoric acid a blue or yellowish green color, and the resulting phosphoric acid cannot be used in applications where purity is essential. Heretofore, vanadium components present in the phosphoric acid as a result of the wet process have been removed by precipitation with sodium ferrocyanide or sodium chlorate. These processes are not economical on an industrial scale, and require complicated technical operations. In addition, processes for separating or recovering the vanadium components from crude phosphoric acid by an organic solvent extraction have been proposed. However, these known processes require use of a special solvent, operate in a quite complicated manner, or use special chemicals.
A need exists therefore for an effective, simple, and economical method for removing vanadium impurities from crude phoshoric acid.